Essentially, all anti-phosphorylcholine antibodies produced by BALB/c mice bear an idiotype that is indistinguishable from that of TEPC-15 myeloma protein. Treatment of neonates or normal spleen cells with anti-idiotype antibody results in complete tolerance to stimulation by phosphorylcholine-containing antigen. The mechanisms of idotype suppression has been investigated in our laboratory and our recent findings are summarized as follows: 1) anti-idiotype antibodies induce suppressor T cells in vitro from the normal spleen cell populaton in the presence of specific antigen, 2) anti-idiotype antibody also inactivates directly B cells leading to tolerance to antigen or polyclonal B-cell activators. Antigen appears to play an important role in this process of tolerance induction, resulting in early unresponsiveness. Currently, several aspects of investigations for idoitype suppression are underway. For example, kinetics of suppression by anti-idiotype antibody is compared with that by anti-IgM and anti-IgD antibodies. In addition, characterization of suppressor cells involved and their recognizing signals are being investigated.